Many types of input devices are presently available for performing operations in a computing system, such as buttons or keys, mice, trackballs, joysticks, touch sensor panels, touch screens and the like. Touch screens, in particular, are becoming increasingly popular because of their ease and versatility of operation as well as their declining price. Touch screens can include a touch sensor panel, which can be a clear panel with a touch-sensitive surface, and a display device such as a liquid crystal display (LCD) that can be positioned partially or fully behind the panel so that the touch-sensitive surface can cover at least a portion of the viewable area of the display device. Touch screens can allow a user to perform various functions by touching the touch sensor panel using a finger, stylus or other object at a location dictated by a user interface (UI) being displayed by the display device. In general, touch screens can recognize a touch event and the position of the touch event on the touch sensor panel, and the computing system can then interpret the touch event in accordance with the display appearing at the time of the touch event, and thereafter can perform one or more actions based on the touch event.
Touch sensor panels can, in some embodiments, be formed from a matrix of drive lines (e.g., row traces) separated by a dielectric material from a plurality of sense lines (e.g., column traces), with sensors or pixels created at each crossing point of the drive and sense lines. Touch sensor panels can alternatively be arranged in any number of orientations or dimensions, including, but not limited to, diagonal, concentric circles, spiral, three-dimensional, or random orientations. In order to detect and identify the location of a touch on a touch sensor panel, stimulation signals are provided to the drive lines causing the sense lines to generate signals indicative of touch output values. By knowing the timing of the stimulation signals to specific drive lines relative to the signals read out of the sense lines, processor(s) can be used to determine where on the touch sensor panel a touch occurred.
More specifically, the capacitance between various drive and sense lines can be measured and calculated. A touch event can result in a decreased capacitance between these lines. The processor can detect such decreases to determine when and where touch events occur.
When the object touching the touch sensor panel is poorly grounded, touch output values read out of the sense lines may be erroneous, or otherwise distorted. More specifically, various external capacitances, such as the capacitance between the device and ground, or the capacitance between the touch object (i.e., a user's finger) and ground can distort the measurements of the capacitances between various drive and sense lines. The possibility of such erroneous or distorted readings is further increased when two or more simultaneous touch events occur on the touch sensor panel.